The present invention relates to an improvement upon an universal logic controller of the stored program type, that is, a programmable logic controller.
In the area of various kinds of machine and process controls, stored program type universal logic controllers have been widely utilized in recent years in place of conventional wired logic type controllers comprising in combination relays, counters and timers.
As well known, the basic features of this kind of programmable logic controllers are that they can respond to various control specifications by means of the set program which can be easily modified. However, currently known programmable logic controllers are frequently more inconvenient in a certain aspect in actual uses than the wired logic type controllers. This problem will occur when they are used, for example, for control specifications described hereinafter.
As a typical example of a very simple case, the control of an industrial washing machine will be described below. In a controller for performing a combination of operations of an industrial washing machine in a predetermined sequence (start of water filling, stop of water filling, start of draining, stop of draining, detergent filling, drive and stop of pulsator, start of dewatering, stop of dewatering, etc.) it will be assumed hereinafter that part of flow of the control such as "washing time" and "the number of repetitions of the rinse and dewatering" is required to be frequently changed depending upon the kind of clothing to be washed, while the basic control procedure of the whole from the first start of water filling to the last stop of dewatering will remain unchanged.
In this case, in the conventional controller having a wired logic circuit, a variable type timer capable changing the setting time can be used for determining "the washing time" and a preset type counter can be used for determining "the number of repetitions of the rinse and dewatering". Then, these times and counters can be mounted on the most convenient places for workers separately from the controller of the washing machine. If "washing time" is indicated on the timer and "the number of repetitions of rinse and dewatering" is shown on the counter, any ordinary worker who has no knowledge of control engineering may be able to easily and correctly change the operation stated above, as long as he knows how to set the timer and counter. In addition, by just looking at the indications of setting time of timer and the preset value of the counter, "washing time" and "the number of repetitions of rinse and dewatering" currently set can be confirmed by the worker at any time.
On the other hand, if a conventional type of programmable logic controller is adopted as the controller for said washing machine, the control procedure for the washing machine must be expressed in accordance with the predetermined instruction system and written in the program memory of the controller, and the instructions for determining "washing time" and "the number of repetitions of rinse and dewatering" will become part of the program consisting of a number of combined instructions. Therefore, "washing time" and "the number of repetitions of rinse and dewatering" can be naturally changed by rewriting the corresponding instructions in the program memory. However, it can be said that changing such a program is much more difficult compared with the operation of a timer for "washing time" and a counter for "the number of repetitions" for wired logic type systems described previously.
Instruction systems and setting methods for programs for programmable logic controllers have been improved and simplified in many aspects, and even a kind of controller called "process stepping type", with which any required control operation can be easily realized by workers who have no special knowledge of control circuitry, is already known and available. However, if a certain change or setting of its program is needed, workers have to know at least the operating sequence of such controllers and the coded instructions.
Thus, if time data of "timer instructions" or numerical data of "counter instructions" which is only a very small part of the whole control program are going to be revised in order to change the "washing time" or "the number of repetitions of rinse and dewatering", it is first required to find the step numbers of instructions in program for "washing time" and "the number of repetitions" by checking the program lists or by sequentially monitoring the program. Then, an area in the program memory corresponding to the step number previously found has to be located and new instructions for the change have to be entered there. This operation is the same as a program setting operation, and any simple error during key operation for changing the instructions may alter other instructions not related to "washing time" and "the number of repetitions". Thus, even the operation for changing only a small portion of instructions cannot be performed quickly and accurately by workers who have no knowledge of the instruction system and the operating method of the controllers. That is, such conventional programmable controllers are not suited to such washing machines for which "washing time" and "the number of repetitions of rinse and dewatering" have to be frequently changed by ordinarly workers in response to the kinds of clothing to be washed. Even though a program indicator is provided in a conventional controller, only the instructions of the step being executed or the instructions of the step designated by the monitoring operation are indicated in the coded form, so that currently set "washing time" and "the number of repetitions of rinse and dewatering" cannot be easily confirmed. In addition, setters for "washing time" and "the number of repetitions" for such controller cannot be installed at places which are very easily accessible to workers.